Plasticized polyvinyl butyral (PVB) sheet is used in the manufacture of laminate structures such as, for example: windshields for vehicles including automobiles, motorcycles, boats and airplanes; homes and buildings; shelving in cabinets and display cases; and other articles where structural strength is desirable in a glass sheet. In many applications, it is desirable that the laminate be transparent and colorless, or at least have very low color. Undesired or unintended color in a glass laminate can be a result of impurities from various sources. In some cases, color can occur in the PVB interlayer. Color in a PVB sheet can result from several sources in the PVB resin, or from the manufacturing process. For example, color can result from instability of the PVB resin, from impurities, or from other additives present in the PVB composition. Color in a PVB sheet can develop during storage of the PVB, or be caused by process conditions to which the resin is subjected.
In a conventional PVB sheet manufacturing process, additives can be included to protect PVB from developing color. Examples of such additives are antioxidants, such as octylphenol, and ultra violet light stabilizers. In some commercial applications, it has been found that combinations of additives can be required for satisfactory results. For example, conventional PVB sheet compositions can include, Tinuvin® P, Tinuvin® 123 in addition to Tinuvin® 326 to obtain satisfactory light stability in the sheet. Using combinations of additives can add additional expense and complexity to the manufacturing process, particularly if a change in the additives must be made. For example, in some instances changing the composition used in a chemical or polymer manufacturing process can have unexpected results. The unexpected results can sometimes be unwanted.
Conventional PVB sheet typically includes a plasticizer in order to increase the flexibility and processibility of the PVB sheet. Generally, the higher the concentration of plasticizer, the more flexible the sheet. Various plasticizers are conventional in the manufacture of PVB, and include such plasticizers as: diesters of polyethylene glycols such as triethylene glycol di(2-ethylhexanoate) (3GO) and tetraethylene glycol diheptanoate (4G7), for example. Typically, plasticizer is included in amounts of greater than 30 pph, based on the total dry weight of the resin. Depending upon the application, as well as other factors, highly plasticized PVB can have as much as 60 pph of plasticizer.
In some special applications, however, it can be desirable to include small amounts of plasticizer so that a stiff PVB sheet can be obtained. One problem with using low plasticizer concentrations is that the PVB composition may require exposure to higher temperatures in order to extrude the resin and/or to shape the extruded sheet for use in the application for which it was intended. However, when a conventional PVB resin is exposed to such high temperature, color can develop in the sheet thereby making it unusable in applications where clarity and high laminate transparency are critical.
It can be desirable in the art of manufacturing PVB laminate articles, to prepare a stiff PVB sheet, wherein the sheet includes a low concentration of plasticizer, by a process wherein the PVB composition does not develop a high color level upon exposure to high temperatures during extrusion or lamination.
It can also be desirable to obtain PVB sheet by a process wherein the color of the sheet can be improved by proper selection of additives used in the process.